Dogs have a biological need to walk.
Wild dogs and ancestors of our domestic dogs would travel for hours a day to guard their territory and to obtain resources. Up until a couple of decades ago our dogs were working or free to explore outside rather than sitting on the couch all day, as they do now. Many of our domestic dogs were bred for a certain purpose, such as hunting, herding, or other types of work. Even mutts will retain biological drives to physical engage in an activity, and walking is crucial for fulfilling that need.
Walking is physically stimulating and provides many of the same benefits that walking does for humans, such as weight loss or maintenance, stronger bones, muscles, and joints, improved attention, and disease prevention. Just because your dog has the energy to burn, it does not mean dogs will motivate themselves to exercise. Even if you have a large yard, it takes active participation from their owners to motivate dogs to move. It’s great if you engage in other activities with your dog, such as fetch, running, or agility, but a plain old walk can be extremely beneficial as well.
Walking provides mental stimulation, introducing them to new places, people, dogs, scents, sounds, and sights. Being let out into the same backyard does not provide the same enrichment as an active environment like a park. Dogs are motivated to travel and explore. Allowing them the opportunity to do this makes for a happy, satisfied dog. Ultimately, walking is an activity you should be doing with your dog. If you do not have the time or energy, hiring a dog walker can be a great way to give your dog the opportunity without having to add another item to your to-do list.